This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. High-level modulation of the B0 external magnetic field is desirable in millimeter-wave ESR studies over a wide range of problems in chemistry and biology where the linewidths are often large (100G) given that the signal to noise ratio (SNR) is proportional to the modulation amplitude (mod amp) for mod amps less than or approaching 1/5 of the line width. For high-level modulation of B0 in our 170 and 240 GHz spectrometer bridges, we have designed an extremely wideband power amplifier which is capable of stably driving off-tuned resonant loads which exhibit a large reactive component. Conventional audio-frequency power amplifiers, which are sometimes pressed into service as modulation coil drivers, are not well-suited for this type of service. Generally, they are inefficient and poorly characterized in the region above approximately 50 KHz, especially when driving highly capacitive or inductive impedances. The ACERT modulation amplifier design, in contrast, overcomes those limitations. It operates in either constant-current (CC) or constant-voltage (CV) modes, and can produce up to 50 watts of output power at frequencies from d.c. to 150KHz. Utilizing a commercially-available h.v. hybrid operational amplifier, this wideband power driver design is simple, low-cost and robust, but also features high linearity and stability. Its output section includes provisions for detection and signaling of abnormal output conditions, such as clipping due to, e.g., mistuning of the modulation coil network. Operation in CC mode minimizes amplitude drift in the detected spectrum due to modulation coil self-heating and permits direct modulation with a constant magnetic field generation coefficient, when a fixed input-to-field ratio is required. CV mode permits use of the amplifier in other laboratory applications such as piezoelectric actuator driver for CD-ESR. Having prototyped and tested sections of the modulation driver amplifier, our next step in the project plan is to design a compact package for the complete amplifier and construct several integrated units for HF, and possible CD-ESR use, during the coming year.